The Astonishing Adventure of Jane Smith (Musaicum Vintage Mysteries). Patricia Wentworth

Jane Smith is a feisty young woman but looks a lot like her dear cousin Renata Molloy. One day, while sleepwalking, Molloy reaches where she shouldn't—in the den of Number One, the most notorious mind behind a secret organisation. Now it is up to Jane to protect her cousin and don on the mantle of a female spy to thwart Number One's plan and save the day! Excerpt: "The air was heavy with the smoke of bad tobacco and the fumes of a very indifferent gas fire. There was a table in the middle of the room, and some dozen of the men were seated at it. The rest stood in groups, or leaned against the walls. Of the four who formed the Inner Council three were present. Most of the Delegates had expected that the head of The Council, the head of the Federated Organisations, that mysterious Number One whom they all knew by reputation and yet had never seen in the flesh, would be present in person to take the chair…"